Electrical and electronic circuits generally follow Ohm's law, V=IR, with circuit components having material-specific electrical resistivity. It would be advantageous if the resistance of one or more circuit components were easily tunable so that circuit parameters such as voltage, current, power, or resonance frequency were tunable. While there are highly magnetoresistive materials having electrical resistivity sensitive to applied magnetic field, these materials typically require continuous power to maintain a change in resistivity.
One well-known technique for altering the electrical resistivity of a material is to vary the temperature. However, temperature control is cumbersome and requires sustained power to maintain a desired temperature. An elevated operating temperature is also likely to cause undesirable changes in other circuit components.
It is known that a magnetic field can affect the resistivity in magnetoresistive (MR) materials. The "magnetoresistance" (MR) of a material is the resistance R(H) of the material in an applied field H less the resistance R.sub.o in the absence of an applied field, i.e. MR=R(H)-R.sub.o. The resistance difference MR is typically normalized, by dividing by R(H) and expressed as a MR ratio in percent: EQU MR ratio=(R(H)-R.sub.o)/R(H)
Conventional magnetic materials (e.g., permalloy) typically have a positive MR ratio of a few percent. Recently, relatively large values of MR ratio were observed in metallic multilayer structures, e.g. Fe/Cr or Cu/Co. See, for instance, P. M. Levy, Science, Vol. 256, p. 972 (1992); E. F. Fullerton, Applied Physics Letters, Vol. 63, p. 1699 (1993); and T. L. Hylton, Science, vol. 265, p. 1021 (1993). More recently, very large changes in electrical resistivity have been induced in certain types of MR materials, such as the colossal magnetoresistance (CMR) compounds. See Jin et al., Science, Vol. 264, p. 413 (1994); Jin et al., JOM, Vol. 49, No. 3, March 1997, p. 61; and G. A. Prinz, Physics Today, Vol. 4, p. 58 (1995). While resistivity changes by many orders of magnitude are obtained in CMR, such changes typically require the use of very high magnetic fields of 1 tesla or higher. But application of magnetic fields greater than 0.1 tesla (1000 Oe) is not practical for device circuits. Although magnetic fields can be amplified by using soft magnetic poles or cores, continuous power is required for such amplification.
Accordingly, there is a need for circuits wherein one can obtain large and programmable changes in electrical resistivity using practical, low magnetic fields, and without requiring continuous power.